Problem: Simplify the following expression: $p = \dfrac{3y}{7} - \dfrac{y}{3}$
Solution: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $7$ and $3$ $\lcm(7, 3) = 21$ $ p = \dfrac{3}{3} \cdot \dfrac{3y}{7} - \dfrac{7}{7} \cdot \dfrac{y}{3} $ $p = \dfrac{9y}{21} - \dfrac{7y}{21}$ $p = \dfrac{9y -7y}{21}$ $p = \dfrac{2y}{21}$